Hi all,

got a short enquiry. I’ve got these premade 3mm LEDs that already have the resistor soldered to run under 12v DC instead of 3v.

This is a flashing LED and the circuit is pretty simple and already has a green constant LED, red flashing and pink flashing.

On the circuit there is a motor that when it operates, the pink flashing Led should turn on and start flashing. The Green LED is constantly on to show that the circuit has power and it works all the time.

Problem is when the motor has some load, the pink LED stops flashing or flashes randomly. I’ve found this issue with the pink LEDs, flashing and non flashing. Should be something to do with the colour?

Wherever I connect the pink LED on the circuit, the same issue occurs. Even controlled by a relay, different power supply, regulated or unregulated, more amps etc.

Could someone shed some light on how I could fix this?

Regards,

Steve

 

If the motor current causes the supply voltage to jump up and down, messing up the LED flashing circuit then add a capacitor parallel to the LED to smooth its voltage. Try 47uF.

The flashing LEDs in my solar garden lights have a capacitor parallel with the LED.

 

Thank you for your help. I will try this and let you know.

Think the pink/UV LED colour requires higher voltage to operate properly.

 

Awesome, the trick worked with the capacitor!! Thank you very much for your help

 

First, a clearer picture of what you're describing is needed. From what you've said I've come up with a diagram below (called "AS DESCRIBED"). If that is not correct please correct it.

I highly respect @Audioguru but in this case more clarity on what he's proposing is needed. Below you see "Prescribed Fix (1)" and "Prescribed Fix (2)". Of course there are different ways of connecting a cap (or 2 or 3) so I've drawn two figures. Here's why I disagree. If the motor is loaded down the cap will provide the needed current to carry the LEDs AND the motor. If the motor is bogged down the cap will quickly deplete and you'll be back to the same problem.

Something thing you didn't tell us is how many amps your 12 volt supply is capable of and how many amps the motor draws. Motor current is important as well because at startup the motor will draw more current than when it's up and running. It's running current is also important because it tells us how much current is available for the rest of the circuit. In my illustration I'm showing a 12 VDC 2 amp source and the motor drawing 1 amp when running. That leaves plenty of amperage for the LEDs, assuming they're running on 20 mA (milliamps). But if the motor bogs down under a load then the motor will draw more amperage. If the motor starts drawing 1 1/2 amps (1.5 A or 1500 mA) then there's not enough amperage for the LEDs to properly light. IF (and it's a BIG IF) the supply can't deliver enough current then no matter what you do you're going to continue to experience the same issue. In my illustration labeled "Proper Fix" (so labeled to differentiate mine from the others) shows an additional diode. This will prevent the capacitor from feeding into the circuit when the motor bogs down. FOR A SHORT TIME! The capacitor can only deliver so much power before it becomes depleted - and once again you're back to the same issue.

It's my opinion that the correct solution to the problem is a bigger power supply. One capable of higher amperage. The LED's don't run on "Voltage" they run on "Current". And if the motor draws most of the current then there's not enough left to run the LEDs. Even though I show a "Proper Fix" I don't honestly think that's the correct fix. It's only "Proper" based on Audioguru's capacitor solution. It's good for a moment but not good for a longer duration of motor loading. AND the diode before the cap is going to drop some voltage. If you go that route you will likely need to use a Shotkey diode. They drop much less voltage than a regular diode will.

 

Since the motor draws a much higher current than the LEDs and it runs fine without a huge expensive capacitor then I would filter the LEDs.
Since two of the LEDs each flash their own pattern then each LED needs its own capacitor parallel with it and its flashing circuit. The green LED does not flash so it does not need a capacitor. Do it like this:

 

Yes that’s correct, I did explain on my original post that a higher current PSU made no difference. The whole circuit draws less than 500mA when motor is operating and the PSU used is 1A. Not sure what’s the startup current as I don’t have an oscilloscope to calculate the voltage drop.

Even a 2A PSU made no difference as there was where my thought went initially.



The problem occurred mainly when the motor has load. On normal/automatic operation, the motor doesn’t operate more than 1-2s.

Since it’s a flashing LED, the current supplied by the cap sustains it even at prolonged period (manually) under load.
The motor is geared by the way.

I was a bit baffled as well and that’s why I posted the question here. Another theory is that on the UV led they used the same resistor as the other colours, but after some research I found that the UV needs a bit more volts to operate. (4.5 instead of 3.3v).
The LEDs came prefabricated with the resistor in place to operate at 12v. Tried from other sources and got the same results.

Thank you very much for your input, it’s very useful

 

OK, just guessing now - could it be caused by Back EMF (BEMF) from the motor itself? This is probably my last post on this topic because it's not likely I could contribute anything useful from here on out. But if Audioguru's approach solves the problem I'd like to know that as well. Always good to learn something new.

 

Yes it did solve the issue. 47uf cap did the trick. Probably other capacitors would do but didn’t check as it worked immediately.

The BEMF is something I didn’t think about to be honest. On a brushless motor it didn’t create this issue with the LED.